Why did GM use plastic for the 1946 Chevrolet front parking light lenses? normally they use glass??

I bought my '46 Fleetmaster in 1973 from the second owner and all appeared to be original. It had and still has glass parking light lenses.

Below is just a small sample of the glass '46-8 lenses available on eBay. The bottom one states it's an NOS glass lens in the original Guide box. There are many more that look like glass but the description did not state glass or Lucite. I'll have to check when I get home but I think the lenses in my '48 Cabriolet are glass too.

http://www.ebay.com/itm/1948-1947-1946-chevy-parking-light-lenses-glass-/122364035662?hash=item1c7d77024e:g:4tMAAOSwx6pYne2X&vxp=

http://www.ebay.com/itm/1942-1946-1947-1948-CHEVROLET-FRONT-PARKING-LIGHTS-1-/222530784493?hash=item33cfde4ced:g:aAkAAOSwTM5YuGb3&vxp=

http://www.ebay.com/itm/1947-1948-Chevy-Passenger-Car-LH-Parking-Light-Lamp-Lens-NOS-/291950009893?hash=item43f9939e25:g:U3cAAOSw5cNYINN1&vxp=mtr